1. Field of the Invention
This invention relates to a method of determining the optimum electric discharge machining conditions for a machining electrode and a workpiece based on predefined machining parameters, and an electric discharge machining controller.
2. Description of the Prior Art
Since the electrode used in electric discharge machining machines an area larger than itself (machining overcut), the machining is conducted using an undersized electrode fabricated to a smaller size than that of the machining dimensions required by the workpiece. As electric discharge machining is more time consuming than other machining methods, it is generally divided into a number of stages between roughing and finishing so as to increase machining efficiency. In the first stage, machining is conducted under rough conditions for approximately obtaining the desired machined configuration. This is known as roughing. In the next stage or stages, called semifinishing, the machining current is reduced and relative movement between the electrode and the workpiece is effected. Finally, finishing is conducted to obtain the required machining dimensions and degree of machined surface roughness.
In determining the roughing conditions in the initial roughing stage, the operator adopts the maximum machining electric current allowable in light of the electrode undersize, machining depth and machining area, and then decides, with reference to a data book or the like, the machining depth and the stock to be left for finishing and required for obtaining the final desired surface roughness.
Among the machining conditions that come into play during roughing, the machining current has the strongest influence on the electric discharge machining overcut and the finishing allowance left for subsequent finishing. The roughing conditions are therefore greatly affected by the amount of electrode undersize. This is because the maximum current usable in roughing is determined almost entirely by the electrode undersize. Therefore, after calculating the maximum permissible machining current from the machining area, the operator checks whether the electrode undersize is sufficiently greater than the machining overcut that can be expected at the calculated machining current value. If it is, roughing is conducted using machining conditions based on the calculated value. If the electrode undersize is smaller than the expected machining overcut, the operation refers to a data book and selects machining conditions which will create a machining overcut that is smaller than the amount of electrode undersize. The general practice in this case is to set the machining current on the low side in consideration of the maximum electric discharge machining overcut, including the machining produced by secondary discharges through the chips (called machining deformation).
This setting of the rough machining current at a relatively low level in light of the anticipated machining overcut and the machining deformation lengthens the time required to complete the roughing operation. Although it is theoretically possible to machine at maximum machining current by using a sufficiently undersized electrode, this method cannot be applied when there are restrictions on the machining configuration or when the electrode has already been fabricated. In such cases there is no alternative but to determine the machining current with reference to the electrode undersize.